Imaging based measurement is widely in use. One can find imaging-based measurement systems based on triangulation, interferometry or 2D projection. However, these devices are subject to good alignment, critical calibration procedures and typically sensitive to lighting conditions. This limits the ability to use these imaging instruments on a portable basis.
It is preferable to have a non-contact dimension measurement device that is inexpensive and portable for certain applications. For instance, in a steel rolling mill, the roll closure (the gap between two rolls) is a critical parameter relating to the quality of the finished products. While fixed location dimensional measurement instruments are commonly available to the steel mills, they are too expensive to be installed after every rolling stand. Therefore, these instruments are typically installed at the finishing end, after the last rolling stand. Nevertheless, there is a need to verify the dimensions after every stand on a periodic basis, such as after a new machine setup or after a predetermined time period (e.g., every 30 minutes). The difficulty involved in taking such a measurement is that the object to be measured is not only very hot, typically ranging from 700° C. to 1,250° C., but is also moving. It is not practically possible to stop the object for measurement because a constant no-moving contact can damage the rolls. There are two practices now for taking a measurement for the hot object while moving. The first practice is to use a hand-held caliper for those objects moving at a relatively slow speed, such as after a roughing mill. In this practice, an operator must bring a caliper into contact with the hot moving object to obtain the measurement. As may be appreciated, this practice is somewhat dangerous and potentially inaccurate. The other practice, for objects moving at a faster speed, such as after an intermediate mill, involves using a piece of a specially treated wood. The wood piece is held against the hot, moving object, thereby allowing the hot object to burn a portion of the wood piece into the shape of the object. The worker then measures the burned portion to obtain the dimension of the object. A portable non-contact dimension measurement device could find itself valuable in this application.
Further, it is known to use a hand-held optical instrument for measuring a distance to an object (e.g., such as a range finder); however, such a device does not help when the dimension of the object is desired. In addition, a LIDAR (light detection and ranging) system is known in the art, which is capable of scanning with a laser beam for topographical data, but such a system is expensive and not portable.
One challenge in providing a portable two-dimensional (2D) projection based measurement device is that the projection size in an image may be greatly affected by the setup and/or configuration, or known as the spatial relationship between the imaging sensor, typically a camera, and the object to be measured. Portability in such a measurement device thus prevents knowing in advance the object-to-measurement device spatial relationship.
Another challenge in providing a portable measurement device is the unknown nature of the lighting conditions and setup the may exist at a time and place when a measurement is desired to be taken. In particular, those of ordinary skill in the art know that in general, the quality of an image may greatly affect the information that can be extracted from the image, which for present purposes would greatly affect measurement results. For example, it is undesirable to have any portion of the object to be measured so bright that it will saturate (i.e., image brightness over the limit) the imaging system. Such saturation effect will cause the boundary detection algorithms, used to define the boundaries of an object to derive the dimensions, to fail. Furthermore, such saturation may cause a spill over effect, with excessive photons flooding adjacent image pixels (i.e., in typically used imaging technologies). This will result in a swollen object in the image, thereby causing faulty measurements. It is also undesirable to have the object too dark, in that the imaging sensors will behave non-linearly, due to the so-called dark current effect. Dark current effect is caused by long wavelength photons penetrating into the substrate of the imaging sensor. When the brightness is low (e.g., less than 15% of the full scale, for instance), the dark current effect may distort the image, particularly the object boundaries. In contrast to a portable device, in a fixed installation, a typical imaging system may experience the benefit of a fixed and well-controlled lighting environment to ensure proper exposure and working range. However, as suggested, this outcome is almost impossible for a portable device because the lighting conditions and setup cannot be known in advance. Even today's digital cameras, equipped with automatic electronic shutter, aperture and flash lights, cannot fully address some extreme cases, such as imaging a steel bar at 1,000° C.
There is therefore a need and value to design a measurement device, capable of portable use and capable of measuring object dimensions based on 2D projection, that eliminates or minimizes one or more of the problems or shortcoming described above.